The present invention relates in general to a digital signal code transmission method and in particular to a method of transmitting a digital signal arranged in a series of blocks or frames each including an error detection code and an error correction code in addition to information codes and which method is advantageously suited for protecting the quality of reproduced sound from degradation due to code errors in a system destined for transmitting or processing digital audio signals.
There is known a so-called compact disc signal system which represents a typical one of the type of signal transmission system in which a digital information word to be transmitted is divided into a plurality of information symbols and parity words for code error detection and code error correction are combined with every predetermined number of information symbols.
In the compact disc system, a digital information word is constituted by an audio signal sample quantized to 16 bits which are grouped into two information symbols one of which includes eight high-ordered (more significant) bits with the other including eight low-ordered (less significant) bits, to thereby constitute the error detecting code and the error correcting code. Generally, in the signal transmission system under consideration, four symbols of a Reed Solomon code destined primarily to be used for the correction of error are generated or created and added to 24 information symbols which include the high-ordered and low-ordered symbols in pairs for every information word of twelve samples.
Further, in the compact disc system, a first signal frame composed of 24 information symbols and four first parity symbols is subjected to an interleaving operation on a symbol basis, whereby a Reed Solomon code of four symbols to be used primarily for error detection is generated for 28 symbols in total including 24 information symbols and 4 first parity symbols derived from the first signal frames differing from one another and added as a second parity symbol, to thereby constitute a second signal frame. The signal sequence arranged in this manner is generally referred to as the cross interleave code. The Reed Solomon code has an error detecting and correcting capability. Thus, the signal reproduced by the compact disc system undergoes error detection by the second parity symbol for every second signal frame. Among the errors, those which are able to be corrected by using the second parity symbol naturally undergo correction. On the other hand, an error which can not be corrected is marked by a flag indicative of the presence of the symbol suffering the error.
Here, a brief elucidation will be made of the cross interleave code so as to provide a better understanding of the invention. FIG. 1 of the accompanying drawings is a schematic view for illustrating preparation of a cross interleave code, in which the numbers of input channels are taken along the ordinate with the serial numbers of the codes taken along the abscissa. As will be seen in the figure, the signal frames of the two varieties are prepared in the arrangement illustrated. More specifically, there are prepared a second signal or code frame consisting of information or data words D.sub.0,0, D.sub.1,1, D.sub.2,2, D.sub.3,3, D.sub.4,4, D.sub.5,5, D.sub.6,6 and D.sub.7,7 and check words Q.sub.8,8 and Q.sub.9,9 and a first signal or code frame consisting of information or data words D.sub.4,0, D.sub.4,1, D.sub.4,2, D.sub.4,3, D.sub.4,4 and D.sub.4,5 and check words P.sub.4,6, P.sub.4,7. In this case, only one word is permitted to make an appearance in duplicate in both the first and second code frames. By virtue of such arrangement, the errors of which correction is impossible in the second code frame such as, for example, D.sub.0,0, D.sub.2,2 and D.sub.4,4 indicated by hatched areas can be corrected in the first code frames including D.sub.0,0, D.sub.2,2 and D.sub.4,4 respectively. In this way, the error detection and correction of codes can be effected with a high efficiency. In case the Reed Solomon code is employed in the first and second code parts, the code which has undergone the cross interleave operation is referred to as the cross-interleave Reed Solomon code (CIRC).
A hitherto known interleave system for preparing the cross interleave code will be described.
First, an audio analog signal is quantized to a predetermined number of bits, e.g. 16 bits, at a predetermined sampling frequency, e.g. 44.1 kHz.
The 16-bit data thus obtained can be encoded in two ways. First, the 16-bit data can be encoded as it is. Second, the 16-bit data can be processed on the basis of a predetermined number of bits, e.g. 8 bits. In the instant description, it is assumed that the 16-bit data is divided into eight high-ordered (more significant) bits and eight low-ordered (less significant) bits, wherein the data of the eight high-ordered bits is represented by S.sub.U with the data of the eight low-ordered bits being represented by S.sub.L. Further, the sequential orders of the data S.sub.U and S.sub.L are, respectively, represented by symbols S.sub.iU.sup.j and S.sub.iL.sup.j, where i takes ordinal values of 0 to 5 for each value of j. For example, the data is in the order of S.sub.0.sup.j, S.sub.1.sup.j, S.sub.2.sup.j, S.sub.3.sup.j, S.sub.4.sup.j, S.sub.0.sup.j+1, S.sub.1.sup.j+1, and so on. Thus, S.sub.iU.sup.j represents data comprising the i-th high-ordered bit of j-th group. An example of interleaving the codes of the data structure mentioned above is illustrated in FIG. 2 of the accompanying drawings.
Referring to FIG. 2, in the first coding, the data S.sub.0U.sup.7 ; S.sub.0L.sup.7, S.sub.1U.sup.7 ; S.sub.1L.sup.7 ; S.sub.2U.sup.7 ; S.sub.2L.sup.7, S.sub.3U.sup.7 ; S.sub.3L.sup.7, S.sub.4U.sup.7 ; S.sub.4L.sup.7, and S.sub.5U.sup.7 ; S.sub.5L.sup.7 are encoded as information words to generate or create the check words P.sub.0.sup.7, P.sub.1.sup.7, P.sub.2.sup.7 and P.sub.3.sup.7. For these words, interleave is effected in a sequence of S.sub.0U.sup.0, S.sub.0L.sup.1, S.sub.1U.sup.2, S.sub.1L.sup.3, S.sub.2U.sup.4, S.sub.2L.sup.5, 2.sub.3U.sup.6, S.sub.3L.sup.7, S.sub.4U.sup.8, S.sub.4L.sup.9, S.sub.5U.sup.10, S.sub.5L.sup.11, P.sub.0.sup.12, P.sub.1.sup.13, P.sub.2.sup.14 and P.sub.3.sup.15. For the interleaved words, the error check code CRC.sup.16 is generated through the second encoding.
By virtue of the interleave operation mentioned above, a succession of errors possibly generated in a second code frame in the course of recording and/or reproduction will be dispersed in the first code frame upon decoding thereof. Thus, the decoding of the second code frame can be accomplished with an improved efficiency.
However, if the error correction is rendered impossible due to a succession of errors involved in the interleave operation, the twelve data S.sub.0U.sup.0, S.sub.0L.sup.1, S.sub.1U.sup.2, S.sub.1L.sup.3, S.sub.2U.sup.4, S.sub.2L.sup.5, S.sub.3U.sup.6, S.sub.3L.sup.7, S.sub.4U.sup.8, S.sub.4L.sup.9, S.sub.5U.sup.10 and S.sub.5L.sup.11 become erroneous.
Describing a method of decoding the code series mentioned above, the individual signal symbols are rearranged to the structure of the second signal frame through deinterleaving, wherein the erroneous symbols remaining uncorrected are corrected with the aid of the first parity symbol. Further, by taking advantage of the error detecting capability of the Reed Solomon code, the erroneous symbols remaining undetected and/or erroneously corrected symbols still suffering error can be detected and corrected by resorting to the second parity symbols in many applications. In this manner, the data whose errors can not be corrected even by resorting to the first parity symbol are outputted together with the error detection flag constituted precedingly by the second parity symbol. Furthermore, when the error whose detection is missed by the second parity symbol is detected and found uncorrectable, a fresh error detection flag is added before being outputted.
The individual information symbols processed in the manner mentioned above are combined on a two-symbol basis so as to constitute the information words which are original audio signal samples and subjected to a digital-to-analogue (A/D) conversion to be outputted as the audio signal. At that time, if at least one of the symbols corresponding to the high-ordered and the low-ordered information words has an error detection flag attached thereto, error concealment is performed through a mean value interpolation or previous value holding (zero-th order interpolation) procedure with a view to preventing a click from being generated, to thereby minimize deterioration in the reproduced sound quality.
The system for performing detection and correction of code errors by dividing information word into a plurality of the symbols is advantageous in that the capability of error correction can be enhanced for a given code redundancy, and that the number of components, such as registers required for arithmetic operation for the error correction can be decreased. As a result the scale of the circuit can be correspondingly reduced, because the number of bits forming a signal word (symbol) to be processed as a basic element is small.
Next, consideration will be given to the concept elucidated above as applied to a system where the code error generated is of a burst nature as in the case of a digital audio recording/reproducing system employing a magnetic tape, by way of example. It is further assumed for purpose of illustration that a CRC code which is made use of in the error detection adopted in a PCM audio recorder using a VTR (video tape recoder) is utilized as the second parity word, and that a b-adjacent code is employed as the first parity word for the error correction. In the PCM audio recording apparatus using a VTR, six signal information words each of 14 bits are combined with two words of the b-adjacent code of 14 bits and subsequently subjected to an interleave operation so as to be combined with the CRC code of 16 bits. The signal which has undergone the error detection for every second signal frame with the aid of the CRC code is so as to be rearranged in the first signal frame. Since two words of the b-adjacent code are attached, it is possible to correct all the errors of two words within the first signal frame. More specifically, so far as the errors in the second signal frame are not more than two frames, the error in the first signal frame is less than two words, which means that all the errors can be corrected. However, in case the error is over three frames in the second signal frames, the errors included in the first signal frame will amount to three or more words, resulting in a word which is left uncorrectable.
Here, it is noted that by dividing the signal information word of 14 bits into two symbols each of 7 bits, four symbols of the b-adjacent code each of 7 bits are available for the same given redundancy. Similarly, when the CRC code of 16 bits is employed in the second signal frame, an equivalent error detection capability can be obtained. However, since the b-adjacent code of four symbols is capable of correcting errors up to four symbols, errors of less than four frames can be corrected in the second signal frames, increasing the error correcting capability.
In case each of the information symbols of the first frame comprises a combination of the high-ordered symbol and the low-ordered symbol of each information word as in the case of the code employed in the compact disc system, as shown in FIG. 3, there may arise uncorrectable errors of the information words for the reasons mentioned below. In FIG. 3, directions indicated by inclined or diagonal interconnecting lines indicate the second frames for error detection, while the first frames for error correction are arrayed in the horizontal direction. Each of the information words is constituted by two symbols, e.g. S.sub.Ou.sup.(2) and S.sub.0L.sup.(2), S.sub.1U.sup.(2) and S.sub.1L.sup.(2), and so on.
It is assumed that, in a CRC code, errors are detected in the five second signal frames in which those symbols are placed which are to constitute the first signal frames such as S.sub.0U.sup.(4), S.sub.0L.sup.(4), S.sub.1U.sup.(4), S.sub.1L.sup.(4), S.sub.2U.sup.(4), S.sub.2L.sup.(4) and so on, inclusive of the second signal frame of S.sub.0U.sup.(2), S.sub.0L.sup.(3), S.sub.1U.sup.(4) and so on and the second signal frame of S.sub.0U.sup.(0), S.sub.0L.sup.(1), S.sub.1U.sup.(2), S.sub.1L.sup.(3), S.sub.2U.sup.(4) and so on, and that an error is found in the four second frames in which those symbols have been placed which constitute the first signal frame of S.sub.0U.sup.(0), S.sub.0L.sup.(0) and so on. On these assumed conditions, in the two first signal frames S.sub.0U.sup.(0) and S.sub.0U.sup.(1), respectively, correction can be made as the error is of four symbols. However, in the three first signal frames comprising S.sub.0U.sup.(2), S.sub.0U.sup.(3) and S.sub.0U.sup.(4), respectively, five symbols suffer errors which are thus uncorrectable. As a consequence at least the six symbols S.sub.0U.sup.(2), S.sub.1U.sup.(2), S.sub.0L.sup.(3), S.sub.1L.sup.(3), S.sub.1U.sup.(4) and S.sub.2U.sup.(4) shown in FIG. 3 are outputted as the uncorrectable symbols. Besides, many of the symbols (not shown) within the three second signal frames also suffering an error will be outputted as uncorrectable symbols. The individual information words containing these symbols are processed as error information words which are concealed when they are outputted. So far as only the portion illustrated in FIG. 3 is concerned, errors of six symbols will give rise to six error-concealed words inclusive of one word of S.sub.0U.sup.(2) and S.sub.0L.sup.(2). These error-concealed words also provide a cause for generation of distortion in the reproduced signal in the digital audio system and involves deterioration in the quality of reproduced sound, in case the error-concealed words amount to a non-negligible number. The prior art systems described above are disclosed, for example, in the Odaka et al U.S. Pat. No. 4,413,340 issued Nov. 1, 1983 (corresponding to G.B. Patent No. 2,076,569A and the DE-OS No. 3,119,669) and Doi et al's U.S. Pat. No. 4,355,392 issued Oct. 19, 1982.